A typical spark plug includes a metal shell to which a ground electrode is connected and which holds a center electrode in an insulating manner with an insulator interposed therebetween. The spark plug ignites an air-fuel mixture in a combustion chamber of an internal combustion engine by causing a spark discharge between the ground electrode and the center electrode. The spark plug becomes unable to cause a spark discharge when the insulation resistance decreases due to accumulation of carbon, generated by incomplete combustion or the like, on the surface of the insulator and when the applied voltage becomes lower than the required voltage (voltage at which the spark discharge occurs). Accordingly, a self-cleaning spark plug has been proposed (Japanese Unexamined Patent Application Publication No. 2012-79417). In the self-cleaning spark plug, two surfaces having different outer diameters are arranged adjacent to each other in an axial direction of the center electrode. Carbon that has adhered to the insulator is burnt off by a micro discharge starting at the boundary (edge) between the two surfaces.
To ensure sufficient heat resistance, corrosion resistance, and thermal conductivity, the center electrode includes an electrode base member and a core member embedded in the electrode base member. The electrode base member has a cylindrical shape with a bottom, and has high heat resistance and high corrosion resistance. The core member has a thermal conductivity higher than that of the electrode base member. The electrode base member and the core member are integrated with each other by forging. The edge is formed by compressing and thinning a part of a front end portion of the electrode base member by forging.
However, when the edge is formed by thinning a part of the electrode base member by forging, the core member embedded in the electrode base member is also compressed. Therefore, there is a problem that the cross section of the heat-conducting core member is reduced.